1. Field of the Invention
The present invention generally relates to a liquid crystal electro-optical (display) device, and particularly relates to a liquid crystal electro-optical (display) device in which light-interrupting patterns are provided on one of a pair of substrates constituting a liquid crystal panel on which substrate thin-film transistors (TFTs) are formed.
2. Description of the Related Art
The liquid crystal display device is widely used in a projection display apparatus and as a display device of a portable (or lap-top) information processing apparatus. Recently, attempts to use the liquid crystal display device as a high-resolution color display device of a stationary (or desk-top) information processing apparatus have started.
FIG. 10 shows an example of a projection liquid crystal display apparatus. Referring to FIG. 10, light emitted from a high-intensity metal halide lamp 61 is passed through a filter 61a, changed in optical path by a mirror 62, and then partially reflected by a dichroic mirror 63. The light reflected by the mirror 63 is again reflected by a mirror 64, and passed through a condenser lens 65 and a liquid crystal display panel 66. The light outputted from the liquid crystal display panel 66 passes through dichroic mirrors 67 and 68, and reaches a projection lens 69.
On the other hand, part of the light passed through the dichroic mirror 63 is reflected by a dichroic mirror 70, passed through a condenser lens 71 and a liquid crystal panel 72, and then reflected by the dichroic mirror 67. The light reflected by the mirror 67 reaches the projection lens 69 through the dichroic mirror 68.
The light passed through the dichroic mirror 70 is passed through a condenser lens 73 and a liquid crystal display panel 74, then reflected by a mirror 75 and the dichroic mirror 68, and finally projected by the projection lens 69.
Among several types of liquid crystal display devices, the active matrix liquid crystal display device, in which an individual pixel is driven by a thin-film transistor (TFT), is suitable for attaining the above-mentioned high-resolution color display. The active matrix driving method can eliminate crosstalk between pixels which may occur in the passive matrix driving method, and hence can provide superior display performance. In the active matrix driving method, TFTs are arranged on one of the glass substrates that constitute a liquid crystal panel and each TFT controls a voltage applied to a corresponding, transparent pixel electrode.
In the active matrix liquid crystal display device, portions of the liquid crystal layer existing over the pixel electrodes receive drive electric fields generated by means of the TFTs and hence serve for on/off-control of light transmission. However, portions of the liquid crystal layer right above the TFTs are not given any drive electric fields, projection light, back light, or the like may leak through those portions. To improve the display contrast ratio, it is necessary to minimize the light that leaks through the portions where the TFTs are formed. To this end, conventionally, light-interrupting patterns are formed on the substrate that is opposed to the TFT-bearing substrate in the portions where the TFTs are formed. However, in this configuration, precise alignment is needed between the two substrate so that the light-interrupting patterns cover the corresponding TFTs, as a result of which the liquid crystal panel assembling step takes a long time. If wider light-interrupting patterns are formed to facilitate the alignment operation, there arise such problems as a decrease in display brightness.
To solve the above problems, the present inventors investigated a configuration in which light-interrupting patterns are formed on the TFT-bearing substrate, and produced experimental models of such a liquid crystal display device.
FIG. 11 is a plan view of a liquid crystal display panel experimentally produced by the inventors. Referring to FIG. 11, a plurality of generally U-shaped polysilicon patterns 1 are formed corresponding to respective pixels of the liquid crystal display device on one of the two glass substrates opposed to each other to constitute a liquid crystal panel. Connection pads 1a and 1b are formed at both ends of each polysilicon pattern 1. A plurality of aluminum gate bus patterns 2 are formed parallel with each other on the same glass substrate so as to cross over the polysilicon patterns 1 through a SiO.sub.2 film serving as a gate insulating film. In each polysilicon pattern 1, the portions on both sides of the portions where the polysilicon pattern 1 crosses the gate bus pattern 2 are doped with an n-type or p-type impurity, so that channel regions of TFTs are formed corresponding to the crossing points of the polysilicon pattern 1 and the gate bus pattern 2. For each U-shaped polysilicon pattern 1, a pair of n-channel or p-channel TFTs are formed so as to be connected to each other in series.
Further, a plurality of aluminum data bus patterns 3 are formed parallel with each other in the vertical direction as viewed in FIG. 11 on the polysilicon patterns 1 so as to correspond to the respective connection pads 1b. Each data bus pattern 3 is connected to the corresponding connection pad 1b via a contact hole, and drive current is supplied via the connection pad 1b to a p-type or n-type drain region that is formed in the polysilicon pattern 1. On the other hand, the connection pad 1a is connected to a transparent pixel electrode through a light-interrupting pattern or an aluminum pattern (described below).
In the above configuration, a light-interrupting mask 5 is formed corresponding to the gate bus patterns 2 and the data bus patterns 3. The light-interrupting mask 5, which is made of an opaque metal such as Ti, interrupts light that leaks through openings between adjacent pixel electrodes.
FIG. 12 is a sectional view taken along line Z-Z' in FIG. 11. Referring to FIG. 12, the polysilicon pattern 1 is formed on a glass substrate 10 and covered with a first-layer insulating film 6. The data bus pattern 3 is formed on the insulating film 6, and connected to the connection pad 1b of the polysilicon pattern 1 via a contact hole 6a that is formed through the insulating film 6.
The data bus pattern 3 is covered with a second-layer insulating film 7, and the light-interrupting mask 5 is formed on the insulating film 7 so as to correspond to the data bus pattern 3. The light-interrupting mask 5 is covered with a third-layer insulating film 8, and an ITO pattern 9 as a transparent electrode is formed on the insulating film 8. The transparent electrode pattern 9 is connected to the connection pad 1a of the polysilicon pattern 1 via another contact hole (not shown).
Although not shown in the sectional view of FIG. 12, the gate bus pattern 2 is formed on the substrate 10 in the same level as the polysilicon pattern 1, and crosses over the polysilicon pattern 1 at the channel regions of the TFTs.
In FIG. 11, the data bus pattern 3 is drawn so as to be shifter leftward from the polysilicon pattern 1. This simply indicates that the data bus pattern 3 is formed on the polysilicon pattern 1 as shown in FIG. 12.
In the above configuration, the portions of the light-interrupting mask 5 extending in the vertical direction as viewed in FIG. 11 (i.e., in the column direction) interrupt light that leaks out passing by both side edges of the data bus pattern 3 which extends in the vertical direction. Since the data bus pattern 3 possibly interacts with the transparent electrode pattern 9 via a capacitance, it cannot be made so wide as to overlap the electrode pattern 9; a gap g should necessarily be formed in between. The light interrupting mask 5 interrupts light that leaks through the gap g.
In the configuration of FIGS. 11 and 12, the light-interrupting mask 5 can be formed on the substrate 10 on which the TFTs are formed, by part of a series of steps for forming the TFTs. Therefore, substantially ideal alignment can be attained between the light-interrupting mask 5 and the TFTs, whereby the conventional step of accurately aligning the substrate 10 with the opposed substrate on which a light-interrupting pattern is formed can be omitted.
However, when a liquid crystal display device having the configuration of FIGS. 11 and 12 was actually produced, a problem was found that the metal pattern as the light-interrupting mask 5 was likely short-circuited with the data bus pattern 3, making it difficult to increase the yield of the liquid crystal display device.
This is considered due to a phenomenon that insufficient step coverage of the insulating film 7 with respect to the data bus pattern 3 causes short-circuiting between the light-interrupting mask 5 and the data bus pattern 3, as illustrated in FIG. 13. Similar short-circuiting likely occurs between the light-interrupting film 5 and the transparent electrode pattern 9. Since the light-interrupting mask 5 is a continuous member extending vertically and horizontally as shown in FIG. 11, such short-circuiting causes a line defect.